1. Field of the Invention
The present invention relates generally to optical fibers, and particularly to a method for identifying optical fibers which exhibit elevated polarization mode dispersion (PMD).
2. Technical Background
PMD is an important factor in the design of state-of-art fiber optic transmission systems. The effect of PMD in fiber systems is evident when, after propagating a sufficient distance in the network, one digital pulse may spread in the time domain and become indistinguishable from a nearby pulse. The pulse spreading from PMD can introduce errors into the data transmission, effectively limiting the transmission rate of the pulses or the maximum distance of the concatenated fiber medium. Polarization mode dispersion (PMD) can be a major limitation for high data rate optical communications systems and is thus an important fiber attribute that fiber manufacturers monitor closely. One notorious aspect of fiber PMD is that the fiber having the highest PMD value (sometimes referred to as the fiber PMD “outlier”) dictates the performance of the overall system. Although fiber outliers usually represent a very small percentage of the whole population of newly made fibers, the effect of such fibers can be very detrimental. A single fiber with significantly elevated PMD increases the PMDq specification, a parameter that is very sensitive to the highest PMD in a population of fibers. Fiber PMD failure identified at a cable plant can also incur significant financial loss to the fiber makers.
Recent progress has been made in detecting fibers with elevated PMD using polarization optical time domain reflectometry (POTDR). See: B. Huttner, B. Gisin, and N. Gisin, “Distributed PMD measurement with a polarization-OTDR in Optical fibers”, J. Lightwave Technology, 17, 1843 (1999); X. Chen et al. “Method of evaluating fiber PMD using polarization optical time domain reflectometry”, US2004/0084611; P. Fayolle et al., “Polarized Lightwave reflectometry method (POTDR)”, US2004/0046995; and S. Tanigawa et al., “Optical fiber polarization-mode dispersion measurement method and measurement device”, PCT/JP2003/009175. Fibers with elevated PMD can be identified and removed so that the PMD uniformity of the fibers and the compliance with the specification can be ensured. However, a common feature of the above known methods is that they are effective only when the fiber has elevated PMD over a sufficiently long length to permit detection by those schemes. But, at the very local level the metric that is used to judge the fiber PMD often fluctuates significantly. It is desirable for a measurement scheme that is capable of detecting major PMD failure at (or within a) very local regime, even down to a matter of a few meters, because major failure of fiber PMD can occur within a very short fiber length. In some instances, PMD failures have been detected and linked to gas bubbles or air lines, sometimes called seeds, in the fiber perform, between the interfaces of different preform manufacturing steps, which can occur more often in the manufacture of fibers with complex refractive index profiles made with multiple steps, although similar conditions can occur in the manufacture of relatively simple step index fibers. The extent of the elevated PMD due to such seeds can range from a meter and more along the length of the fiber.